Electro-optically controlled measurement probe system

ABSTRACT

A needle-shaped terminal, i.e., a probe tip, is brought into extremely close proximity with an object being measured. This probe tip is formed from an optically active material and in itself is electrically non-conductive, so that it has no electrical influence whatsoever on the object being measured. When optical pulses are beamed onto this probe tip, the probe tip becomes an electric conductor and a current flows between it and the object being measured, so that the electric potential of the object can be measured. This facilitates previously impossible measurements of high-speed electric waveforms. It also facilitates high spatial resolution monitoring and control of probe tip position and characterization of measurement sites, and enables highly reliable measurements to be made inexpensively.

TECHNICAL FIELD

This invention is utilized for semiconductor research and manufacture,and it relates in particular to techniques for measuring the operatingstate of high-speed integrated electronic circuits. It is utilized forthe observation and measurement of extremely small, precise structures,and as a precision measuring instrument capable of measuring theelectrical characteristics of electric circuits on which extremelyminute mechanical structures have been formed, and where the spacingbetween adjacent circuits is extremely small. This invention is suitedfor utilization in semiconductor device manufacture and research, and ithas been developed in order to make electrical measurements of theoperating state of high-speed integrated electronic circuits.

BACKGROUND TECHNOLOGY

The frequency of the signals that are dealt with in the field ofelectronics has recently reached 250 GHz, and the current status ofhigh-speed electrical measuring technology is that the methods employedto observe these high-speed electric waveforms have not kept up withtechnical progress. Furthermore, advances in the miniaturization ofcircuit elements have led to a situation in which neither the temporalresolution nor the spatial resolution of electric measuring instrumentshas kept up with current technical progress.

An established and representative means of observing high-speedphenomena such as the high-speed operating state of microcircuitelements is the sampling oscilloscope. In addition, studies haverecently been made of EO sampling, a technique which utilizes theelectro-optic effect in electro-optic crystals (see Kamiya, T. andTakahashi, M., "Electro-optic sampling using semiconductor lasers," OyoButsuri, Vol. 61, No. 1, p. 30, 1992; and Nagatsuma, T., "Measurement ofhigh-speed devices and integrated circuits using electro-optic samplingtechnique," IEICE Trans. Electron., Vol.E-76C, No. 1, January 1993).Given that it has become comparatively easy in the field of lasertechnology to obtain optical pulses in the subpicosecond range, opticalsampling techniques seek to use such laser pulses for sampling electricsignals. These techniques are faster than conventional electronicmeasurements, and can provide direct, non-contact measurements ofelectric potential at a desired point on a circuit under test, withoutleading out the signal. In other words, with this technique the electricpulses used for sampling in a sampling oscilloscope are replaced withoptical pulses.

Another method for making high spatial resolution direct measurements ofelectric potential at a desired point is the electron-beam tester (seePlows, G., "Electron-beam probing," Semiconductors and Semimetals, Vol.28 (Measurement of high-speed signals in solid-state devices), Chap. 6,p. 336, Ed. Willardson, R. K. and Beer, Albert C., Academic Press,1990). Electron-beam testers are a powerful means of observing electricsignals inside ICs for diagnosis and analysis of IC operation.

Scanning tunnelling microscopes and atomic force microscopes are amongthe devices which have recently undergone rapid development and comeinto widespread use as instruments for making high spatial resolutionobservations of the surface shape of objects under test. Because thesedevices are able to provide three-dimensional images at ultra-highspatial resolutions corresponding to the atomic scale, they are verywell suited to the observation of the surface shape of semiconductorintegrated circuits and the like. Bloom et al. have recently proposed amethod for measuring the electric potential of an object under testusing an atomic force microscope (AFM) (see Hou, A. S., Ho, F. andBloom, D. M., "Picosecond electrical sampling using a scanning forcemicroscope," Electronics Letters, Vol. 28, No. 25, p. 2302, 1992). Inthis method, a high-speed electronic circuit is used as the object beingmeasured by an ordinary AFM, and a repulsive or attractive force isproduced between the AFM probe and this object in accordance with theelectric potential at the point being measured. This force gives rise toa minute displacement of the probe position. The method proposed byBloom et al. involves detecting these minute displacements in order tomeasure changes with time in the electric potential at the point beingmeasured.

However, the temporal resolution of a sampling oscilloscope is limitedby the speed at which measurements can be carried out. This is dependenton a time constant which is determined by the width of the electricpulses used for sampling and by the resistance and capacitance of themeasuring system. Moreover, because the signal being measured is led outfrom the measurement point via a cable or waveguide, it ends up beingdisturbed, with the result that there are problems regarding itsreliability as well.

In the EO sampling technique, it is difficult to measure the absolutevalue of the signal, and there are practical problems relating to, amongother things, the methods employed to provide high spatial resolutionmonitoring and control of probe position.

Electron-beam testers have a low temporal resolution and cannot beapplied to the evaluation of ICs that use high-speed transistors. Theyalso have the inconvenience of requiring a high vacuum as themeasurement environment.

The temporal resolution of scanning tunnelling microscopes and atomicforce microscopes has a limit that is set by the response speed of theprobe, which is a mechanical system, and it is therefore difficult touse these devices for the measurement of high-speed electric waveforms.

Accordingly, a new means of measurement is required for the accurateevaluation of electric waveforms in large-scale integrated electroniccircuits.

This situation may be looked at from another point of view as well.Namely, in prior art devices, only the pursuit of sampling speed hasbeen regarded as important, and hardly any consideration has been givento positional control. The prior art will be explained with reference toFIG. 35, which is a block diagram of a prior art device. Object beingmeasured 1 lies on testing stage 133, and the part of the object that isto be measured is set, by the operation of testing stage positioncontroller 132, in the vicinity of probe tip 51, which is supported byprobe arm 21. Next, after probe tip 51 has been brought into contactwith object being measured 1, adjustment in the height direction iscarried out by vertical position controller 130 which is formed from apiezoelectric element or the like, and the optimum measurement positionis determined. Light is beamed onto probe tip 51 from light source 92.If an electric potential is present at the measurement site on object 1,the refractive index of the electro-optic crystal will change due to theelectro-optic effect, whereupon the direction of polarization of thelight reflected back from probe tip 51 will change from the direction itwould have had if no electric potential had been present. The amount ofchange is detected by an optical system comprising wave plate 99,polarizer 97 and photodetector element 11, and is input to electricmeasuring instrument 60, which results in a measurement of the electricpotential at the site being measured.

Prior art devices of this kind have the following problems. Namely, whenthe object being measured is an integrated circuit or the like where thethickness varies from place to place, every time the measurement site ischanged it will be necessary to repeat the operations of bringing probetip 51 into contact and positioning it, with the result that (1)measurements take a long time, and (2) the large mass of the part thatholds probe tip 51 means that there is a strong chance of physicallydamaging the circuits of the object being measured.

Thus, in prior art devices, only the pursuit of sampling speed has beenregarded as important, and hardly any consideration has been given topositional control.

The present invention has been devised in the light of this situationand meets the need for high temporal and spatial resolution measurementof high-speed electric waveforms at any measurement point on or in anintegrated circuit. It is applicable to faster and more minute objectsof measurement, and its purpose is to provide a more reliable high-speedelectric measuring instrument and probe for the instrument, and anatomic force microscope which will serve as a high-speed electricmeasuring method and measuring instrument.

A further purpose of this invention is to provide an electro-opticmeasuring instrument which can perform high-precision position control.

DISCLOSURE OF THE INVENTION

This invention provides an electric measuring instrument which has aprobe means that is brought into close proximity with an electronicdevice being measured, and a means which measures electric current byway of this probe means; the electric measuring instrument beingcharacterized in that at least part of the probe means comprises anoptically active material, and in that there is provided a means whichbeams pulses of light through this optically active material, the pulsesof light altering the conductive state of the material. Theaforementioned optically active material should be in a non-conductivestate in the measurement environment, and may be a group IV, group III-Vor group II-VI semiconductor.

In this specification, the aforementioned means which measures electriccurrent may be construed as being a means which measures electricpotential. Because this invention is aimed at application to electronicdevices, current is measured in order to measure potential. Furthermore,in this specification, the terms "probe means" or "probe" indicate adetecting part comprising a probe tip that is in close proximity to theobject being measured, and an arm which supports this probe tip.

In order to make ultrahigh-speed, ultrahigh spatial resolutionmeasurements of changes with time in the electric field of an objectunder test, light in the form of short pulses is beamed at the objectand at the probe tip positioned in its vicinity, the probe tip being aneedle-shaped terminal, whereupon an electric current will be produced.This current will flow only when light is irradiating the probe tip, andwill depend on the voltage between the test object and the probe tip. Bymeasuring this current, the electric potential of the object under testcan be sampled. In other words, although the probe tip is broughtextremely close to the object under test, by itself it is electricallynon-conductive, and so the resistance and capacitance, etc., of themeasuring system have no electrical influence whatsoever on the testobject. However, when an optical pulse irradiates this probe tip formedof an optically active material, the probe tip becomes a conductor and acurrent will flow between it and the object under test, with the resultthat the electric potential of said object can be measured.

It does not matter whether the probe tip and the object under test arekept apart at a distance of the order of atoms, or are brought intocontact. If they are apart, the current will flow by means of the tunneleffect or field emission. If they are in contact and a barrier hasformed at their interface, current will flow as a result of tunnelling,while if a barrier has not formed, an ordinary current will flow. Evenif there is contact, the influence on the object under test can be madesatisfactorily small if the resistance presented by the probe currentpath is sufficiently large and the capacitance of the optically activeregion when non-conductive is sufficiently small. In this case, the sizeof the detected current will be determined by this resistance value, andwill be proportional to the electric potential of the object under test.

A high-speed electric measuring instrument constituted in this mannerfacilitates previously impossible measurements of high-speed electricwaveforms. It also facilitates high spatial resolution monitoring andcontrol of probe tip position and characterization of measurement sites,and enables high reliability measurements to be made inexpensively.

It is desirable for the aforementioned probe means to comprise a probetip, and for part or all of this probe tip to be an optically activematerial.

It is also feasible for the aforementioned probe means to be providedwith a probe tip made of an electrically conductive material, and forthe optically active material to be interposed in the electrical pathwhich connects the probe tip made of this conductive material to theaforementioned measuring means.

It is also feasible to provide a synchronization circuit whichsynchronizes the aforementioned means which produces optical pulses andthe operation of the aforementioned electronic device being measured.

The aforementioned optical pulses may be periodic optical pulses.

This invention also provides a probe which has a probe tip that isbrought into close proximity with an electronic device under test, theprobe transmitting, to an electric measuring instrument, the electricpotential at the point with which the probe tip is in close proximity;and the probe is characterized in that an optically active material isinterposed in the electrical path from the aforementioned probe tip tothe aforementioned electric measuring instrument, and in that it has ameans which introduces light into this optically active material, thelight activating this material to an electrically conductive state. Itis desirable for at least part of the aforementioned probe tip to beformed from an optically active material.

An arm which mechanically holds the aforementioned probe tip may also beprovided, and the aforementioned means which introduces light maycomprise an optical waveguide provided along this arm.

It is also feasible for the aforementioned optical waveguide to be anoptical fiber, for a metal coating to be applied to this optical fiber,and for this metal coating to form part of the aforementioned electricalpath.

The aforementioned probe tip may be formed from a conductive material,and the mounting structure which fixes this probe tip to themechanically holding arm may comprise an optically active material. Anoptical waveguide may be provided along this arm.

This invention can also provide an atomic force microscope equipped withthe aforementioned probe. It is desirable for this atomic forcemicroscope to have, as a means for positioning the probe relative to theelectronic device being measured, a means for irradiating the probe withlight and a means for detecting light reflected by the probe. The lightwhich activates the optically active material to an electricallyconductive state may be light emitted from the same light source thatproduces the light used by this positioning means.

This invention also provides a semiconductor integrated circuit whichhas a measuring electrode that is connected to an electric measuringinstrument, the semiconductor integrated circuit being characterized inthat the element being measured and the aforementioned measuringelectrode are connected by an optically active material, and in thatthis optically active material is in a non-conductive state in themeasurement environment, and in that it becomes conductive whenirradiated by light.

Because it has become comparatively easy in the field of lasertechnology to obtain optical pulses in the subpicosecond range, suchlaser pulses are used for sampling electric signals. This enables directmeasurements to be made of electric potential at a point of interest,and such measurements are faster than conventional electronicmeasurements and do not involve leading out the signal. In other words,this technique replaces the electric pulses used for sampling in asampling oscilloscope with optical pulses.

As regards position control of the probe, minute displacements of probeposition can be expressed as displacements in the position of the beamof light which is reflected after striking the probe. A photodetectorelement detects the position of the probe by detecting the displacementof the position at which this reflected light strikes. The position ofthe probe is displaced in accordance with irregularities of the surfaceof the object being measured.

The position of the probe relative to an integrated circuit or otherobject under test which has irregularities, can be controlled with greatprecision by comparing the irregularities that the probe is scanningwith irregularities that are already known. In this way, anelectro-optic measuring instrument capable of high-precision positioncontrol can be achieved by means of a simple constitution.

Embodiments of this invention will now be explained with reference tothe drawings.

BRIEF EXPLANATION OF THE DRAWINGS

FIGS. 1a and 1b are block diagrams of a device according to a firstembodiment of this invention.

FIG. 2 shows an experimental arrangement of this first embodiment.

FIG. 3 shows an experimental arrangement of this first embodiment.

FIGS. 4a to 4c illustrate the measurement principles.

FIG. 5 gives experimental ressults obtained with the first embodiment.

FIG. 6 gives experimental results obtained with the first embodiment.

FIG. 7 gives experimental results obtained with the first embodiment.

FIG. 8 gives experimental results obtained with the first embodiment.

FIGS. 9(a) to 9(d) are experimental results obtained with the firstembodiment.

FIG. 10 gives experimental results obtained with the first embodiment.

FIG. 11 gives measurement results obtained with a device according tothe first embodiment of this invention.

FIG. 12 shows the characteristics of an electronic device being measuredby conventional equipment.

FIGS. 13a and 13b are a block diagrams of a device according to a secondembodiment of this invention.

FIGS. 14a and 14b show a third embodiment of this invention.

FIGS. 15a and 15b are a block diagrams of a device according to a fourthembodiment of this invention.

FIG. 16a and 16b are a block diagrams of a device according to a fifthembodiment of this invention.

FIG. 17 is a perspective view of a sixth embodiment of this invention.

FIGS. 18a and 18b are a perspective views of a seventh embodiment ofthis invention.

FIG. 19 shows the constitution of an eighth embodiment of thisinvention.

FIG. 20 shows the constitution of a ninth embodiment of this invention.

FIG. 21 is a perspective view of a tenth embodiment of this invention.

FIG. 22 shows the constitution of an eleventh embodiment of thisinvention.

FIG. 23 shows the introduction of an optical pulse into a probe.

FIG. 24 is a block diagram of a twelfth embodiment of this invention.

FIGS. 25a and 25b show the constitution of the probe in the twelfthembodiment.

FIG. 26 shows the constitution of the probe in a thirteenth embodimentof this invention.

FIG. 27 shows the constitution of the probe in a fourteenth embodimentof this invention.

FIG. 28 is a block diagram of a device according to a fifteenthembodiment of this invention.

FIG. 29 is a block diagram of a device according to a sixteenthembodiment of this invention.

FIG. 30 is a block diagram of a device according to a seventeenthembodiment of this invention.

FIG. 31 is a block diagram of a device according to an eighteenthembodiment of this invention.

FIG. 32 is a block diagram of a device according to a nineteenthembodiment of this invention.

FIG. 33 is a block diagram of a device according to a twentiethembodiment of this invention.

FIGS. 34a and 34b are a block diagrams of a device according to atwenty-first embodiment of this invention.

FIG. 35 is a block diagram of a prior art device.

OPTIMUM CONFIGURATIONS FOR EMBODYING THIS INVENTION

The constitution of a device according to a first embodiment of thisinvention will be explained with reference to FIGS. 1a and 1b, which areblock diagram of the device. This device is an electric measuringinstrument and is provided with probe 2 which is brought into closeproximity with electronic device being measured 11. It is also providedwith current-to-voltage converter 17, amplifier 18, bandpass filter 19and oscilloscope 10, which together serve as a means for measuring thecurrent flowing in probe 2.

The distinguishing features of this device are that probe 2 comprises anoptically active material, and that optical pulse source 13 is providedas a means for irradiating probe 2 with intense pulses of light whichmake the aforementioned optically active material conductive. Thisoptically active material may be a group IV, group III-V or group II-VIsemiconductor. In a device according to this first embodiment of theinvention, the whole of probe 2 is an optically active material. It waspreviously stated that in this specification, the term "probe" wouldindicate a detecting part comprising a probe tip which is in closeproximity with the object being measured, and an arm which supports thisprobe tip. However, probe 2 in this first embodiment of the inventionshould be interpreted as a probe consisting entirely of a probe tip.

The aforementioned pulses of light are periodic optical pulses. In FIG.1a, optical pulse source 13 and power supply 5 are synchronized bysynchronization circuit 20, while in FIG. 1b there is a means whichsynchronizes the measurement timing of oscilloscope 10 with opticalpulse source 13.

Probe 2 is set up on actuator 16, and the electronic device beingmeasured 11 is set up on actuator 16'. The distance between these twoactuators 16 and 16' is controlled to a preset value by positioncontroller 15.

Next, the operation of this first embodiment of the invention will beexplained with reference to FIG. 2-FIG. 10. FIG. 2 and FIG. 3 showexperimental arrangements of this first embodiment. FIGS. 4a to 4c showthe measurement principles involved. FIG. 5-FIG. 10 show experimentalresults, with time plotted along the horizontal axis and voltage alongthe vertical axis. Experiments based on this first embodiment wereconducted in air using the two experimental arrangements illustrated inFIG. 2 and FIG. 3. In these experiments, for the sake of convenience,the object being measured 1 was formed of an optically active material,and a platinum-iridium alloy (hereinafter, written as "Pt-Ir") was usedas probe 2. Electric potential was measured by activating the opticallyactive material by beaming light in the vicinity of the probe tiplocated at the end of probe 2. In this way, equivalent measurementscould be made of the operating characteristics of a device according tothis first embodiment.

The object test 1 in this first embodiment was a wafer ofsemi-insulating type gallium arsenide (hereinafter, written as "GaAs")or indium phosphide (hereinafter, written as "InP"). An electrode wasmade by sputtering gold (Au) onto the surface of the wafer, and a leadwire was attached by means of a conductive adhesive (silver paste). Toprevent oxidation and/or contamination of the surface of object undertest 1, the water was cleaved immediately prior to measurement and theresulting cleaved surface used as the test object. Pulses of light froman aluminium gallium arsenide (hereinafter, written as "AlGaAs") LEDemitting at a wavelength of 650 nm were used as the light source. LED 3was impedance-matched to 50Ω, and its response time to the input voltagewas 45 ns.

The experiment illustrated in FIG. 2 will be explained first of all.Test object 1 comprising a semiconductor wafer (GaAs or InP) and probe 2are set tip in air and separated by a minute distance. LED 3 is drivenby pulse generator 4, and the vicinity of the tip of probe 2 over testobject 1 is irradiated with light. A DC bias voltage is applied toobject under test 1 by means of power supply 5.

Probe 2 is driven by piezoelectric driver 6, and micro-position controlof the probe is carried out by using a feedback circuit comprisingpreamplifier 7 and operational amplifiers 8 and 9 to detect the tunnelcurrent flowing between probe 2 and object under test 1. Thisconstitutes the same control system as in a scanning tunnellingmicroscope.

The time constant of the feedback circuit as a whole is designed so asto be larger than the time constant of the pulse signal from pulsegenerator 4. However, preamplifier 7 operates fast enough for theaforementioned pulse signal to be fully observable by means ofoscilloscope 10 connected to its output.

Next, the experiment illustrated in FIG. 3 will be explained. FIG. 3shows an experimental arrangement where, instead of applying a DCvoltage to object under test 1, as in FIG. 2, an alternating electricfield was applied.

In FIG. 3, test object 1 and probe 2 are set up as in FIG. 2, i.e., inair and separated by a minute distance. LED 3 is driven by pulsegenerator 4 and the vicinity of the tip of probe 2 over object undertest 1 is irradiated with light.

Object under test 1 is driven by oscillator 25, and frequency counter 31can be used to observe the repetition rates of pulse generator 4 andoscillator 25, and their difference in frequency Δf.

Probe 2 is driven by piezoelectric driver 6, and micro-position controlof the probe is carried out by using a feedback circuit comprisingpreamplifier 7 and operational amplifiers 8 and 9 to detect the tunnelcurrent that flows between probe 2 and object under test 1. The timeconstant of this control system is made the same as in the experimentillustrated in FIG. 2.

The output signal from preamplifier 7 can be observed by means ofdigital oscilloscope 30 via low-pass filter 32. Oscillator 25 isconstituted so that a DC voltage can be applied as an offset.

With this experimental arrangement, a measuring instrument comprisingprobe 2 is set up in contact with or at a minute distance of the orderof atoms (5-10 Å) from object under test 1. This measuring instrument isirradiated with light in the form of short pulses in the vicinity of theregion where test object 1 and probe 2 are opposed, which results in anelectric current being produced and flowing between test object 1 andprobe 2. The electric potential of object under test 1 can be sampled bymeasuring this current, and in this way it is possible to makemeasurements of the operating condition and electric field distribution,etc., of the object under test.

The measurement principles will be explained with reference to FIGS. 4ato 4c. It is assumed that when VD, the potential of the sample deviceunder test (DUT), equals Vt, the potential of the needle-shaped terminalthat serves as the probe tip (see FIG. 4a), the Fermi level of thesample DUT coincides with the center of the forbidden band of the probetip. Under these circumstances, because there will usually be fewcarriers, no tunnel current will flow, nor will field emission occur. Inthis state, if the probe tip is irradiated with light, carriers will becreated and a tunnel current will flow, but only while light isstriking. Depending on the shape of the tunnelling barrier and on otherfactors, a current may also flow due to field emission. The size of thiscurrent will depend on the potential difference between the object beingmeasured and the probe tip, i.e., on VD-Vt (see FIG. 4b and 4c). Itfollows that the potential of the sample DUT can be found by setting Vtappropriately and observing the resulting current.

Utilizing this basic measurement principle, a sampling mode of operationcan be performed by setting the repetition rate of the short laserpulses comprising the irradiating light to a frequency which is veryslightly shifted from the frequency of the electric signal in testobject 1, and measuring the current that flows under thesecircumstances. In this case, the electric signal in test object 1 caneasily be measured by an ordinary measuring instrument such as digitaloscilloscope 30, as a signal with a frequency equal to the difference infrequency between the repetition rate of the above-mentioned opticalpulses and the electric signal: in other words, as a beat component.

An effective way of maintaining the position of the probe tip is toemploy the same sort of means as used in a scanning tunnellingmicroscope (STM) to provide feedback for keeping the average currentconstant. By making the frequency range of this feedback control systemsufficiently narrow and ensuring that (a) the frequency of the electricsignal of test object 1, (b) the repetition rate of the optical pulses,and (c) the resulting beat frequency, are all set sufficiently higherthan this frequency range of the feedback control system, the electricsignal of test object 1 can be measured while maintaining the positionof the probe tip.

Another effective way of maintaining the position of the probe tip is toemploy the technique generally used in an atomic force microscope (AFM):namely, to detect the interatomic force acting between object beingmeasured 1 and the probe tip as a deviation in the position of the probetip, and to provide feedback for keeping this constant.

It is also feasible to make high-precision measurements of potentials onobject under test 1 by applying bias voltages respectively to testobject 1 and/or the probe tip, and adjusting the respective biascurrents. Under these circumstances, silicon or other group IVsemiconductor, gallium arsenide or other group III-V semiconductor, zincselenide or other group II-VI semiconductor, etc., can be used as theabove-mentioned probe tip made of a semiconductor.

The response of the system to optical pulses was confirmed by means ofthe experimental arrangement shown in FIG. 2. The experimental methodemployed and the results obtained will now be explained. Referring toFIG. 2, pulse generator 4 was used to output square pulses of light fromLED 3 while a DC bias was being applied to object under test 1 by meansof power supply 5. These optical pulses irradiated the cleaved surfaceof object under test 1, and oscilloscope 10 was used to measure theresulting output of preamplifier 7, thereby providing observations ofthe response of the tunnel current produced between test object 1 andprobe 2 to the pulses of light. The potential of probe 2 was 0 V, andthe average tunnel current under these circumstances was controlled bythe aforementioned feedback circuit to a value of 1 nA.

Preamplifier 7 converts a 1 nA tunnel current to a voltage of 10 mV, andits frequency range is of the order of 400 kHz. On the other hand, thefrequency range of a scanning tunnelling microscope control system isdetermined by the time constant of the integrating circuit, and in thisfirst embodiment of the invention is set at 300 Hz. A signal in thefrequency region between these two, i.e, in approximately the 1-400 kHzrange, can therefore be applied and detected without disturbing theoperation of the feedback control system.

Various experimental results will now be given. It may be noted that forconvenience, the following measurements were carried out with probe 2and object under test 1 transposed, with a platinum iridium alloy(hereinafter, written as "Pt-Ir") used as probe 2 and GaAs or InP usedas object under test 1. The results obtained (at a frequency of 10 kHz)using semi-insulating type InP as test object 1 are shown in FIG. 5,while the results obtained (at a frequency of 1 kHz) usingsemi-insulating type GaAs are shown in FIG. 6. Square waves 41 and 43 inFIG. 5 and FIG. 6 are the driving voltage of LED 3, while roundedwaveforms 42 and 44 are the tunnel currents. The fact that tunnelcurrent waveforms 42 and 44 are not perfect square waves but are roundedis due to preamplifier 7 having a narrow pass band, and to its timeconstant being approximately 10 μs. This value corresponds closely tothe time constant obtained from these waveforms.

Because the sign of the tunnel current is inverted, the downturnedportions correspond to times when a current is flowing. Due to therebeing an extra offset in preamplifier 7, the origin of the currentappears shifted downwards, but the tunnel current varies between 0 nAand 2 nA, centering on the average tunnel current of 1 nA, which isplausible behaviour.

The results shown here are for the case where the bias voltage appliedby power supply 5 is +2 V DC. Different bias voltages between -5 V and+5 V were tested, and in every case a similar result was obtained:namely, that current flowed only while light was striking themeasurement site. This sort of response of the tunnel current to lightwas also observed when the bias voltage was much smaller than thebandgap of object being measured 1, at practically 0 V.

Next, a description will be given of the experimental method employed inthe experimental arrangement illustrated in FIG. 3 for detecting thebeat between the optical pulses and an AC bias. Referring to FIG. 3, therepetition rate of the optical pulses from LED 3 when driven by pulsegenerator 4 was set at 400 kHz, and the frequency of the AC component ofthe bias voltage applied by oscillator 25 was set at 400 kHz+Δf. A testwas then run to ascertain whether or not a component corresponding tothe beat frequency of was seen in the tunnel current. FIG. 7 showswaveform 45 of the pulses that drive LED 3 (400 kHz). FIG. 8 showswaveform 46 of the AC component of the bias voltage (400 kHz). A DCcomponent of +2 V was superimposed on this as an offset so that thevoltage finally applied to object being measured 1 was a sine wavevarying between +1 V and +3 V. Feedback control was carried out so thatthe resulting average tunnel current became 1 nA.

With this setup, the output from preamplifier 7, which corresponds tothe tunnel current, was measured in a single shot using digitaloscilloscope 30. In order to detect only the beat component Δf, anycomponents in the vicinity of 400 kHz, which is the fundamentalfrequency, were removed by low-pass filter 32. The repetition rate ofthe optical pulses and the frequency of the AC bias voltage weremeasured by frequency counter 31.

Next, FIGS. 9(a) to 9(d) show results obtained when semi-insulating typeInP was used as test object 1, while FIG. 10 shows results obtained whensemi-insulating type GaAs was used. FIG. 9(a)-(d) show, respectively,output waveforms 47-50 obtained when the above-mentioned Δf was set at9.54 kHz, 13.96 kHz, 17.27 kHz and 20.05 kHz. FIG. 10 shows outputwaveform 51 obtained when Δf was 20kHz. In each case, irregularitiescorresponding to the beat can be clearly identified. The number ofirregularities corresponds to each value of Δf, and it will be seen thatthe number of irregularities increases with increasing Δf. The amplitudeis of the order of 1 nA, and this also corresponds closely to the presetaverage tunnel current of 1 nA.

In this first embodiment of the invention, a beat component can also beextracted in the manner described above by varying the optical pulsesirradiating object being measured 1 and the alternating voltage appliedto the object being measured. This is none other than performingsampling. When there is a need to measure a high speed alternatingvoltage signal, it will be possible to make the desired measurement bymeans of the above-mentioned sampling, provided that ultrahigh-speedoptical pulses are employed. With this invention, high-speed samplingthat has previously been impossible by electronic measurement can becarried out easily. The industrial value of this is extremely high.

This first embodiment of the invention has been described in terms ofcases in which metal was used as probe 2 and a semiconductor wafer wasused as object being measured 1. However, similar results were obtainedin the opposite case, namely, where a semiconductor was used as probe 2and metal and/or semiconductor was used as object being measured 1.Furthermore, the region in which the tunnel current is produced has aminute area, and therefore good results were obtained even when theobject being measured had a minute pattern. It may also be noted thatthe tracking system was satisfactorily fast even when picosecond orsubpicosecond optical pulse widths were employed. Approximately the sameeffects were also obtained when a group II-VI semiconductor was used.Although this first embodiment of the invention has been illustrated interms of the use of semi-insulating type semiconductors, a similareffect was obtained (albeit sensitivity was somewhat lower) when p-typeor n-type semiconductors were used.

The results of measurements carried out using the circuit shown in FIG.1b are given in FIG. 11. Periodic pulses of light were generated fromoptical pulse source 13 by periodic excitation, and this was done insuch manner that the phases of these pulses of light and of theoperation of electronic device under test 11 diverged little by little.In specific terms, the trigger signal applied to electronic device undertest 11 from power supply 5 was set at 100.000 MHz, while the repetitionrate of optical pulse source 13 was set at 100.001 MHz. In other words,electronic device under test 11 was set so as to operate repeatedly at arepetition rate fd of 100.000 MHz, while the repetition rate fp of theoptical pulses was set at 100.001 MHz. The wavelength of the opticalpulses was 670 nm and their pulse width was 80 ps. The horizontal axisin FIG. 11 is the time axis and the vertical axis is the voltage level(drawn to an arbitrary scale). One division on the time axis is 200 μs,which means that the repetition period that appears in FIG. 11 is 1000μs. In other words, the repetition rate is 1 kHz. This is equivalent tothe difference frequency between the repetition rate of the opticalpulses and the frequency of the trigger signal, namely:

    Δf=|fp-fd|=1kHz

FIG. 12 illustrates the characteristics of a comparison, showing theresult obtained when the actual operation of electronic device 11 wasmeasured directly using, not the method according to this invention, butan extremely expensive measuring instrument (in this case, a YokogawaDL8100). The horizontal axis in FIG. 12 is the time axis and thevertical axis is the voltage level (drawn to an arbitrary scale). Onedivision on the time axis of FIG. 12 is 2 ns. It will therefore be seenthat what is observed in FIG. 12 is a repetition period of 10 ns, or inother words, the response characteristics of electronic device undertest 11 when triggered by a trigger signal with a repetition rate of 100MHz.

Comparison of FIG. 11 and FIG. 12 shows that, since the repetition ratein FIG. 11 is 1 kHz, what in effect has been achieved is to observe theresponse waveform of electronic device 11, which operates at the 100 MHzrepetition rate shown in FIG. 12, by means of an inexpensive ordinaryoscilloscope capable of measuring 1 kHz. Side-by-side comparison of FIG.11 with FIG. 12 shows that even the fine response curve in FIG. 11(i.e., the response on a time scale of the order of 300 ps) is in goodcorrespondence with that shown in FIG. 12, which indicates that faithfulmeasurements can be performed by means of a device embodying thisinvention.

The extremely expensive measuring instrument which was mentioned aboveand which was used for making the comparison measurements, is the mostsophisticated of the instruments to which the inventor's laboratory hasdirect access, and it is an instrument capable of measuring waveforms ofthe order of 300 ps (in frequency terms, this corresponds to 3 GHz). Aninstrument capable of making faithful measurements of waveforms withshorter periods than this is simply not available, and even if it were,it would inevitably be an extremely elaborate and expensive device. Itis not difficult to increase the optical pulse repetition rate in adevice embodying this invention to 10-100 times the 100 MHz which wasused in the foregoing examples of measurement, and with a few clevermodifications of equipment, it will be possible to increase it to over1000 times this value. This means that this invention will make itpossible to observe phenomena occurring on an electronic device atfrequencies in excess of 10 GHz or 100 GHz by using a measuringinstrument such as a quite ordinary synchroscope capable of measurementsof the order of 1 MHz. In other words, it is evident that this inventionenables measurements to be made of signal waveforms in ranges whichpreviously could not be measured. An instrument which is capable ofproviding accurate observations of the state of the signals which occuron semiconductor devices in such ranges will in the future inevitablybecome an essential device, and hence an instrument according to thisinvention will become an extremely useful device in this field.

In the foregoing explanation, in order to obtain measurement resultsthat were easy to understand, the frequency of the trigger signalapplied to electronic device under test 11 using the circuit shown inFIG. 1b was set at 100.000 MHz, while the repetition rate of opticalpulse source 13 was set at 100.001 MHz exactly. In actual measurements,however, measurements which are similar to those described above can becarried out to a sufficient extent in practice by generating opticalpulses at a rate which is nearly equal to the characteristic operatingfrequency of electronic device 11 under test. In other words, even ifthe difference between the two frequencies (Δf) is not accurately fixed,a practical measuring circuit can easily be achieved if synchronizationcircuit 20 is used as indicated in FIG. 1a to synchronize the opticalpulse repetition rate (fp) of optical pulse source 13 and the operatingfrequency (fd) of electronic device 11 under test so that they operateat the same frequency. Alternatively, a practical measuring circuit caneasily be achieved by using a separate frequency generator as indicatedin FIG. 1b to set the optical pulse repetition rate (fp) and theoperating frequency (fd) of electronic device 11 under test to the samefrequency. If an actual measurement system is built to operate in theextremely high frequency regime, the mutual phases of the twofrequencies will inevitably diverge, little by little, due to ambientnoise, small fluctuations of the measurement system, and otherinfluences. In other words, when a practical synchronization circuit 20or a practical frequency generator is used, what will actually happen isthat the repetition rate of the optical pulses and the operatingfrequency of the electronic device being measured will little by littlebecome out of phase (or, the frequencies involved will diverge), withthe result that observations of the sort described above can be carriedout to a sufficient extent in practice even if circuits which willaccurately synchronize the phases and then bring them little by littleout of phase have not been specially provided.

Next, a second embodiment of this invention will be explained withreference to FIGS. 13a and 13b which are block diagrams of this secondembodiment. In this second embodiment, in similar manner to an ordinaryatomic force microscope, the position of probe tip 51 is detected interms of the position of light from continuous-wave laser light source27 after it has been reflected by the back of probe arm 21, and iscontrolled by a position control system that includes optical positiondetector 28. In this case, the position control system and theelectrical measurement system can be completely independent. In similarmanner to the first embodiment of this invention, which was illustratedin FIG. 1, this second embodiment can have the constitution shown inFIG. 13a or the constitution shown in FIG. 13b, according to the meansused to synchronize the measurement timing.

Next, a third embodiment of this invention will be explained withreference to FIGS. 14a and 14b. As shown in FIG. 14a, this invention canbe applied to an electronic device where this is a semiconductorintegrated circuit 42 which has measuring electrode 40 connected to ameasuring instrument, and wherein element being measured 44 andmeasuring electrode 40 are connected by optically active region 46 madeof an optically active material. Furthermore, as shown in FIG. 14b, evenif there is no measuring electrode, high-speed electric signals inelement being measured 44 can be measured by bringing probe 48 intocontact with optically active region 46 made of an optically activematerial which is connected to element being measured 44.

The constitution of a fourth embodiment of this invention will now beexplained with reference to FIGS. 15, which are block diagrams of adevice according to this fourth embodiment. This embodying device is anelectric measuring instrument which is provided with probe 2 which isbrought into close proximity with the electronic device being measured,and which also has current-to-voltage converter 17, amplifier 18,bandpass filter 19 and oscilloscope 10, which together serve as a meansfor measuring the current flowing in this probe 2.

The distinguishing features of this embodying device are that opticallyactive material 12, which is in a non-conductive state in themeasurement environment, is included between probe 2 andcurrent-to-voltage converter 17, and that it is provided with opticalpulse source 13 as a means for producing intense pulses of light whichserve to make this optically active material 12 conductive. Opticallyactive material 12 may be a group IV semiconductor, a group III-Vsemiconductor, or a group II-VI semiconductor. In this fourth embodimentof the invention, probe 2 in its entirety is constituted as a probe tip.

The aforementioned pulses of light are periodic pulses of light, and inFIG. 15a, optical pulse source 13 and power supply 5 are synchronized bysynchronization circuit 20, while in FIG. 15b there is a means whichsynchronizes the measurement timing of oscilloscope 10 with opticalpulse source 13. Optical pulses generated by optical pulse source 13 arebeamed onto optically active material 12 from lens 14 by way of opticalfiber 22.

Probe 2 is installed on actuator 16, and electronic device under test 11is set up on actuator 16'. The distance between these two actuators 16and 16' is controlled to a preset value by position controller 15.

In this fourth embodiment of the invention, the beat component betweenthe optical pulses irradiating electronic device under test 11 and thealternating voltage applied to the electronic device can be extracted byvarying the optical pulses and said alternating voltage. This is noneother than performing sampling. Accordingly, when there is a need tomeasure a high speed alternating voltage signal, it will be possible tomake the desired measurement by means of the above-mentioned sampling,provided that ultrahigh-speed optical pulses are employed. With thisembodying device, high-speed sampling that has previously beenimpossible by electronic measurement can be carried out easily. Theindustrial value of this is extremely high.

This fourth embodiment of the invention has been described in terms ofcases in which metal was used as probe 2 and a semiconductor wafer wasused as electronic device under test 11. However, similar results wereobtained in the opposite case, namely, when a semiconductor was used asprobe 2 and metal and/or semiconductor was used as electronic deviceunder test 11. Furthermore, the region in which a tunnel current isproduced has a minute area, and therefore good results were obtainedeven when the device being measured had a minute pattern. It may also benoted that the tracking system was satisfactorily fast even whenpicosecond or subpicosecond optical pulse widths were employed.Approximately the same effects were also obtained when a group II-VIsemiconductor was used. Although this fourth embodiment of the inventionhas been illustrated in terms of the use of semi-insulating typesemiconductors, a similar effect was obtained (albeit sensitivity wassomewhat lower) when p-type or n-type semiconductors were used.

Next, a fifth embodiment of this invention will be explained withreference to FIGS. 16a and 16b, which are block diagrams of this fifthembodiment. In this fifth embodiment, in similar manner to an ordinaryatomic force microscope, the position of probe tip 51 is detected interms of the position of light from continuous-wave laser light source27 after it has been reflected by the back of probe arm 21, and iscontrolled by a position control system that includes optical positiondetector 28. In this case, the position control system and theelectrical measurement system can be completely independent. In similarmanner to the fourth embodiment of this invention, which was illustratedin FIGS. 15a and 15b , this fifth embodiment can have the constitutionshown in FIG. 16a or the constitution shown in FIG. 16b, according tothe means used to synchronize the measurement timing.

The constitution of a sixth embodiment of this invention will beexplained with reference to FIG. 17, which is a perspective view of thissixth embodiment. This embodiment has probe tip 51 which is made ofmetal and is brought into close proximity with the electronic devicebeing measured, and probe 2 transmits to electric measuring instrument60 the electric potential at the point with which probe tip 51 is inclose proximity.

The distinguishing features of this embodiment are that optically activematerial 53 is interposed between probe tip 51 and electrical path 59which is provided on arm 21 and which culminates in electric measuringinstrument 60, and that there is provided optical waveguide 54 whichguides light to this optically active material 53.

Next, the operation of this sixth embodiment of the invention will beexplained. Optically active material 53 is formed from a group IV, groupIII-V, or group II-VI semiconductor, and electric current can flowthrough this material while it is being irradiated with light. At othertimes, it becomes non-conductive. When no light is striking it,optically active material 53 shown by the hatching in FIG. 17 isnon-conductive, with the result being that probe tip 51 is insulatedfrom electrical path 59. When light is striking it, optically activematerial 53 becomes conductive and sampling can be performed by means ofelectrical measuring device 60. Although metal was used for electricalpath 59 in this sixth embodiment of the invention, it is also feasibleto provide a metal film on the surface of a semiconductor or aninsulating material.

Next, a seventh embodiment of this invention will be explained withreference to FIGS. 18a 18b, which are perspective views of the seventhembodiment. This seventh embodiment uses probe tip 51' formed from anoptically active material instead of metal. Probe tip 51' is irradiatedwith optical pulses by way of reflecting surface 55 which is provided atthe end of optical waveguide 54, which in turn has been provided on arm21. This seventh embodiment of the invention requires fewer componentparts than the sixth embodiment.

Next, eighth and ninth embodiments of this invention will be explainedwith reference to FIG. 19 and FIG. 20, which show the respectiveconstitutions. In the eighth embodiment, as shown in FIG. 19, the coreof optical fiber 56 is taken as optical waveguide 54, and probe tip 51',which is formed of an optically active material, is provided at the endof this waveguide. The cladding of the optical fiber 56 is shown inFIGS. 19 and 20 as reference number 54b. The periphery of optical fiber56 is coated with metal, and this metal coating constitutes electricalpath 52. This embodiment has still fewer component parts than the sixthor seventh embodiments of this invention.

In the ninth embodiment of this invention, as shown in FIG. 20,optically active material 53 is interposed between metal probe tip 51and optical waveguide 54. Although the overall constitution of thisembodiment is the same as that of the eighth embodiment, probe tip 51 isused for objects being measured when a metal tip is appropriate.

Next, a tenth embodiment of this invention will be explained withreference to FIG. 21, which is a perspective view showing the tenthembodiment. Electrical path 52 is connected to metal probe tip 51, theelectrical path being provided on arm 21 and having optically activematerial 53 interposed in an intermediate position. In addition, opticalwaveguide 54 is provided parallel to electrical path 52. This tenthembodiment of the invention has the advantages that the shape ofoptically active material 53 may be an easy-to-fabricate shape such as acube, and that, because it can be set in a variety of positions, thereis a high degree of freedom as regards probe design.

Next, an eleventh embodiment of this invention will be explained withreference to FIG. 22, which shows the constitution of the eleventhembodiment. Probe tip 51' is made of an optically active material and isconnected to optical fiber 56 which has a metal coating on itsperiphery. This is the same as in the previously explained eighthembodiment of this invention. Optical fiber 56 serves as opticalwaveguide 54 and can be freely connected and disconnected at jointsurface 64. Electrical path 52 comprising a metal coating around theperiphery of optical fiber 56 is in contact with conductive fiber holder70, and this conductive fiber holder 70 is connected to electricmeasuring instrument 60. Conductive fibre holder 70 is supported byinsulator 62 made of ethylene tetrafluoride resin. This insulator 62 isjoined to tubular piezoelectric element 61. Probe tip 51' and metalcoated optical fibre 56 thus constitute an integrally replaceablecomponent, and only this has to be replaced when wear occurs. Moreover,by having probe tips 51' of various shapes ready for use, the probe tipcan be changed over in accordance with the object being measured.

Next, an example of introducing an optical pulse into a probe will beexplained with reference to FIG. 23. Probe 2 is supported by probefixing guide 58 as shown in FIG. 23. Although the coupling efficiencyfrom optical fibre 56 into optical waveguide 54 provided in arm 21 isrelatively low, this does not constitute an obstacle, and by polishingthe tip of optical fibre 56 to a point and thereby increasing thisefficiency, improved performance can be obtained. The same can beachieved using a lens.

The constitution of a twelfth embodiment of this invention will beexplained with reference to FIG. 24, which is a block diagram of adevice according to the twelfth embodiment. This device is an atomicforce microscope which is provided with probe arm 21; probe tip 51 whichis fitted to this arm 21 and is brought into close proximity with objectbeing measured 1; optical pulse source 13 which irradiates this arm 21with light; probe position detection circuit 74 which detects theposition of probe tip 51 by means of reflected light from arm 21; andprobe position control circuit 75 which controls the relative positionsof probe tip 51 and object being measured 1.

The distinguishing features of this device are that at least part of arm21 and probe tip 51 is formed of an optically active material which isnon-conductive in the measurement environment but exhibits conductivityas a result of irradiation with light; and that it has a means whichintroduces light that has been emitted from optical pulse source 13 intothis optically active material; and that it has electric measuringinstrument 60 which detects the current or potential between probe tip51 and object being measured 1. In this twelfth embodiment, probe tip 51is formed from an optically active material, arm 21 is formed from aconductive material, and this arm 21 is electrically connected toelectric measuring instrument 60. In addition, the aforementioned meanswhich introduces light comprises a semitransparent mirror which isprovided at the end of arm 21, and probe tip 51 is arranged at theposition where the light that passes through this semitransparent mirrorwill strike.

Next, the operation of a device according to this twelfth embodimentwill be explained. Probe position setting circuit 84 causes probe tip 51to be brought into close proximity with a given position on object beingmeasured 1, in accordance with operator input. Now, the purpose of thistwelfth embodiment of the invention is the electrical measurement of anelectronic device (i.e., object being measured 1 is an electronicdevice). Accordingly, probe tip 51 can also be brought into closeproximity with a desired position on object being measured 1 by havingprobe position detection circuit 74 detect whether or not probe tip 51is tracing the convex region corresponding to a desired wiring positionon the electronic device, taking this detection output as input to probeposition setting circuit 84, and having probe position control circuit75 drive piezoelectric element 83.

Probe tip 51, which has been brought into close proximity with a desiredposition in this manner, is in itself electrically non-conductive, andconsequently does not have any electrical effect on the operatingcharacteristics of the electronic device which is object beingmeasured 1. At this point, if the optically active material which formsprobe tip 51 becomes electrically conductive by irradiation with lightfrom optical pulse source 13, the current present at the point beingmeasured, which is in close proximity to probe tip 51, will flow toprobe tip 51. Electric measuring instrument 60 measures this, with theresult being that electrical measurements can be made of the electronicdevice. Optical sampling in accordance with the period of the pulsesfrom optical pulse source 13 can also be carried out.

Next, the probe in this twelfth embodiment of the invention will beexplained with reference to FIGS. 25a and 25b, which shows theconstitution of the probe. Arm 21 is made of metal. As shown in FIG.25a, the tip of arm 21 is constituted as semitransparent mirror 86. Halfof the light from optical pulse source 13 is reflected and halfirradiates probe tip 51. Because probe tip 51 is formed from anoptically active material, it becomes electrically conductive as aresult of irradiation with light. The optically active material used inthis twelfth embodiment of the invention is a group IV semiconductor.Group III-V and group II-VI semiconductors can also be used. The objectbeing measured was a semiconductor device. For semitransparent mirror86, the surface of a metal thin film was used as a specular surface. Itis also feasible to use a dielectric multilayer film instead of a metalthin film.

Object being measured 1 is moved by micro-position control and probe tip51 is placed in the desired position on the object. At this point intime, because no optical pulses are irradiating probe tip 51, the probetip is non-conductive. Next, optical pulses are supplied from opticalpulse source 13. Part of the light that strikes semitransparent mirror86 passes through it and irradiates probe tip 51. The remaining part isreflected and is incident upon photodetector element 81, whereupon theposition of probe tip 51 is detected. The optically active material thatforms probe tip 51 becomes conductive due to the light irradiating theprobe tip. Under these circumstances, the current at the desiredposition on the semiconductor device that is the object under test willflow by way of probe tip 51 into arm 21 which is made of a conductivemetal, and will be measured by electric measuring instrument 60.Electric signals can therefore be sampled at intervals corresponding tothe emission of successive optical pulses. During the period when nooptical pulse is striking probe tip 51, the probe tip is electricallynon-conductive and has no electrical effect on object under test 1. Highreliability measurement results can therefore be obtained.

As shown in FIG. 25b, it is also feasible to form probe tip 51 of metaland to use an optically active material as intermediary 87 between thisprobe tip 51 and semitransparent mirror 86. The option of a metal probetip 51 is used for suitable test objects.

Next, a thirteenth embodiment of this invention will be explained withreference to FIG. 26, which shows the constitution of probe 2 in thethirteenth embodiment. An optically active material is used asreflecting surface 88 in the location at which semitransparent mirror 86was provided in the twelfth embodiment. In this thirteenth embodiment,probe tip 51 is formed of metal. When the optically active materialwhich forms reflecting surface 88 is irradiated with light from opticalpulse source 13, it becomes conductive, whereupon probe tip 51 becomeselectrically connected to electrical path 52.

Next, a fourteenth embodiment of this invention will be explained withreference to FIG. 27, which shows the constitution of the probe in thefourteenth embodiment. In this fourteenth embodiment, arm 21 is formedof an ethylene tetrafluoride resin or other insulator, and electricalpath 59 is provided on this. Reflecting surface 88 formed from anoptically active material is interposed in the course of this electricalpath. The working of this embodiment is the same as that of thethirteenth embodiment, but because reflecting surface 88 can be providedat any desired position on arm 21, this embodiment has the advantage ofa larger degree of freedom as regards designing arm 21.

The constitution of a fifteenth embodiment of this invention will beexplained with reference to FIG. 28, which shows the constitution of adevice according to the fifteenth embodiment. This device is anelectro-optic measuring instrument which is provided with: probe tip 51,at least part of which is formed from an electro-optic crystal, andwhich is brought into close proximity with object being measured 1;first light source 91 which irradiates this electro-optic crystal withlight; photodetector element 111 which serves as a displacementdetection means which detects the optical displacement of thiselectro-optic crystal from reflected light from this first light source91 after it has passed through the aforementioned electro-optic crystal;and electric measuring instrument 60 which serves as a means formeasuring the potential of object under test 1 from the results of thedisplacement detection performed by this photodetector element 111.

The distinguishing features of this device are that probe tip 51 issupported by arm 21, and that there are provided second light source 92which irradiates probe tip 51 with light; photodetector element 112 andprobe position detection circuit 74, which together serve as a positiondetection means which detects the position of probe tip 51 from lightemitted by the second light source 92 and reflected by probe tip 51; andprobe position control circuit 75 and probe position controller 96,which together serve as a means for controlling the position of probetip 51 relative to object being measured 1, and to which are input theresults of the detection performed by probe position detection circuit74.

Next, the working of this fifteenth embodiment of the invention will beexplained. Object being measured 1 is set up on piezoelectric element83, and the measurement site on this object is set in the vicinity ofprobe tip 51 by operation of probe position controller 96. Scanning isthen carried out with probe tip 51 kept in contact with object beingmeasured 1. Light emitted from light source 92 is reflected at probe tip51 and is then incident upon photodetector element 112. The position atwhich the reflected light is incident upon photodetector element 112will be displaced in correspondence with the positional displacement ofprobe tip 51. Probe position detection circuit 74 measures thisdisplacement and thereby detects the position of probe tip 51. Becausethis fifteenth embodiment of the invention employs, as a holder forprobe tip 51, arm 21 of similar kind to the one used in an atomic forcemicroscope, the mass of the arm is small and the force used on theobject being measured is extremely small. There is therefore hardly anychance of physically damaging the test object in the way described inconnection with the prior art.

Probe tip 51 scans while detecting irregularities on the surface ofobject under test 1, and probe position control circuit 75 positionsprobe tip 51 while making a comparison with a map of the irregularitiesthat has been input beforehand. When the measurement position isdecided, light from light source 91 is beamed onto probe tip 51. If anelectric potential is present at the measurement site on test object 1,the refractive index of the electro-optic crystal will change due to theelectro-optic effect, and the direction of polarization of the lightwhich was emitted from light source 91 and reflected by probe tip 51will change relative to when no electric potential is present. Theamount of change is detected by an optical system comprising wave plate99, polarizer 97 and photodetector element 111, and is input to electricmeasuring instrument 60, which results in a measurement of the electricpotential at the measurement site. Light source 91 emits pulses oflight, and the period of these optical pulses becomes the samplingperiod of the optical sampling.

Next, a sixteenth embodiment of this invention will be explained withreference to FIG. 29, which shows that this sixteenth embodiment differsfrom the fifteenth embodiment in that detection of the position of probetip 51 and measurement of the electro-optic effect are performed using asingle light source 92. Photodetector elements 111 and 112 are splitphotodetectors, with photodetector element 111 comprising receivingparts a and b, and photodetector element 112 comprising receiving partsc and d. Using A, B, C and D to represent respectively the receivinglevels at receiving parts a, b, c and d, (A+D)-(B+C) will be used forposition detection, and (A+B)-(C+D) will be used for the detection ofelectric potential on object being measured 1. Light source 92 emitspulses of light, and using pulses of light for position detection doesnot present any practical problems.

Next, a seventeenth embodiment of this invention will be explained withreference to FIG. 30, which shows that this seventeenth embodimentdiffers from the fifteenth embodiment in that a single light source 92and a single photodetector element 11 are used. Photodetector element 11is a split photodetector comprising receiving parts a and b, and (A-B)will be used for position detection and (A+B) will be used for thedetection of electric potential on object being measured 1.

Next, an eighteenth embodiment of this invention will be explained withreference to FIG. 31, which shows that in this eighteenth embodiment, anoptical waveguide is provided in arm 21. Light emitted from light source92 may be launched directly into the waveguide of arm 21, or it may belaunched via an optical fiber. In other words, the advantage ofproviding a waveguide in arm 21 is that the position at which lightsource 92 is set up can be freely selected. The measurement principleshere are the same as in the seventeenth embodiment, in which a singlephotodetector 11 was provided.

Next, a nineteenth embodiment of this invention will be explained withreference to FIG. 32, which shows that this nineteenth embodiment issimilar to the eighteenth embodiment in that an optical waveguide isprovided in arm 21. However, position detection is performed byphotodetector element 111 and electric potential detection is performedby photodetector element 112. The measurement principles are the same asin the sixteenth embodiment, but in this nineteenth embodiment of theinvention the detection of electric potential is performed using lightthat has returned through the waveguide.

Next, a twentieth embodiment of this invention will be explained withreference to FIG. 33, which is a block diagram of a device according tothis twentieth embodiment. The distinguishing feature of this twentiethembodiment is that probe 2 comprises two parts, namely, electro-opticcrystal 90 and probe tip 51. Probe tip 51 detects irregularities onobject being measured 1. Detection of the position of probe tip 51 iscarried out by light source 92 and photodetector element 112.Electro-optic crystal 90 is brought into close proximity with themeasurement site on object being measured 1 by means of probe tip 51.Electro-optic crystal 90 produces the electro-optic effect in accordancewith the potential at the measurement site. Light from light source 93reaches photodetector element 111 by way of polarizer 97 after beingmultiply reflected within electro-optic crystal 90. In other words,minute displacements which cannot be measured by means of a singlereflection can be measured by means of multiple reflections.Accordingly, the advantage of this twentieth embodiment of the inventionis that it can perform high-sensitivity measurements of electricpotential.

Next, a twenty-first embodiment of this invention will be explained withreference to FIGS. 34a and 3b, which are block diagrams of a deviceaccording to this twenty-first embodiment. In this twenty-firstembodiment of the invention, as shown in FIG. 34a, ground electrode 100is provided on probe tip 51. Ground electrode 100 has a hole formed inits middle and is therefore of a shape that does not obstruct the lightbeam (see FIG. 34b). This enables the absolute value of the electricpotential on the test object to be measured. In the fifteenth to thetwenty-first embodiments of this invention, all the regions where thelight is reflected are treated to form either a mirror or asemitransparent mirror.

As has now been explained, the industrial value of this invention isextremely high, and includes the ability to sample high-speed electricwaveforms in real time, which was impossible with methods based onconventional electronic measurement, plus the ability to provide ameasuring instrument with extremely good temporal and spatialresolution. The advantages of this method may be listed as follows:

(1) subpicosecond temporal resolution,

(2) nanometer spatial resolution,

(3) no crosstalk,

(4) non-contact measurements can be made,

(5) measurements can be made without leading out the signal,

(6) because the various microscopy-related functions of STMs, AFMs andthe like (e.g., the control functions) can be utilized in this inventionwithout modification, the surface of samples and the position of theprobe tip can be monitored with ultrahigh resolution,

(7) the absolute value of signal potential can be measured,

(8) measurements can be made either in air or in a vacuum.

This invention can also be embodied as a semiconductor integratedcircuit which has a measuring electrode connected to a measuringinstrument, and wherein an element being measured and the aforementionedmeasuring electrode are connected by an optically active material. Ifsuch an embodiment is applied to an electronic device, a conventionalmeasuring instrument can be used to make the same electricalmeasurements as those achieved with a device embodying this invention.

This invention meets the need for high temporal and spatial resolutionmeasurement of high-speed electric waveforms at any measurement point onor in an integrated circuit. It is applicable to .faster and more minuteobjects of measurement, and it provides an inexpensive probe for afaster and more reliable electric measuring instrument. It can alsoprovide an atomic force microscope as a more reliable high-speedelectric measuring method and measuring instrument. Because an atomicforce microscope according to this invention is constituted so that partof the light used for detecting the probe tip position irradiates anoptically active material, it is not necessary to have an extra opticalsystem for making the optically active material conductive. In this way,an inexpensive and highly reliable atomic force microscope can beachieved without disturbing the essential working of an atomic forcemicroscope.

This invention enables an electro-optic measuring instrument capable ofperforming high-precision position control to be achieved by means of asimple constitution.

We claim:
 1. An electro-optically controlled probe measurement systemfor measuring high speed signals with a low speed measurementinstrument, said system comprising:a contact probe to be placed withinclose proximity to an electronic device to be measured, said contactprobe comprising:an optically active material, at least one electricallyconductive portion in electrical connection with said optically activematerial, said at least one electrically conductive portion passingelectrical current conducted by said optically active material, and adetector for detecting light reflecting from a surface of said contactprobe; a single light source to provide a single beam of light to saidcontact probe, a first portion of said single beam of light impingingupon said optically active material to alter a conductive state of saidoptically active material, and a second portion of said single beam oflight reflecting from said surface of said contact probe and impingingupon said detector; and probe positioning means to follow an outputsignal of said detector.
 2. An electro-optically controlled probemeasurement system for measuring high speed signals with a low speedmeasurement instrument according to claim 1, said probe positioningmeans comprising:a probe position detection circuit receiving a signalfrom said detector for detecting a position of said contact probe basedon said single beam of light reflecting from said surface of saidcontact probe; and a probe position setting circuit receiving a signalfrom said probe position detection circuit and setting a position ofsaid contact probe based on said signal from said probe positiondetection circuit.
 3. An electro-optically controlled probe measurementsystem for measuring high speed signals with a low speed measurementinstrument according to claim 1, wherein:said optically active materialis one of a group IV semiconductor, a group III-V semiconductor, and agroup II-VI semiconductor.
 4. An electro-optically controlled probemeasurement system for measuring high speed signals with a low speedmeasurement instrument according to claim 1, wherein:said opticallyactive material of said contact probe is a probe tip.
 5. Anelectro-optically controlled probe measurement system for measuring highspeed signals with a low speed measurement instrument according to claim4, further comprising:a probe mount for mounting said probe tip, saidprobe mount comprising an optically active material.
 6. Anelectro-optically controlled probe measurement system for measuring highspeed signals with a low speed measurement instrument according to claim5, further comprising:an optical waveguide for guiding said beam oflight to said optically active material, said optical waveguide beingprovided along said probe mount.
 7. An electro-optically controlledprobe measurement system for measuring high speed signals with a lowspeed measurement instrument according to claim 1, wherein:said at leastone electrically conductive portion is a probe tip.
 8. Anelectro-optically controlled probe measurement system for measuring highspeed signals with a low speed measurement instrument according to claim1, further comprising:a synchronizer circuit for synchronizing a drivenfrequency of said single light source with a frequency of operation ofsaid electronic device.
 9. An electro-optically controlled probemeasurement system for measuring high speed signals with a low speedmeasurement instrument according to claim 8, wherein:said synchronizercircuit sets said driven frequency and said frequency of operation ofsaid electronic device to diverge slightly.
 10. An electro-opticallycontrolled probe measurement system for measuring high speed signalswith a low speed measurement instrument according to claim 1, whereinsaid contact probe further comprises:an optical waveguide for guidingsaid beam of light to said optically active material.
 11. Anelectro-optically controlled probe measurement system for measuring highspeed signals with a low speed measurement instrument according to claim1, wherein:said surface of said contact probe is a semitransparentmirror comprised of at least one of a metal thin film and a dielectricmultilayer film.
 12. An electro-optically controlled probe measurementsystem for measuring high speed signals with a low speed measurementinstrument according to claim 1, wherein said probe measurement systemis applied to an atomic force microscope.